The present invention relates to a semiconductor image sensor for receiving reflected light from an image source illuminated by incident light to convert the received light into a corresponding electric signal.
FIG. 2 is a circuit diagram showing the conventional semiconductor image sensing device. Incident light to the semiconductor image sensor is converted into a corresponding electric signal by an array 1 of photodetecting elements. However, the electric signal converted by the array 1 of photodetectors is rather weak and therefore may be affected by noise generated by a read-out circuit 2. In the circuit 2, each analog switch is formed of a MOS transistor TGn and an inverter In, hence it causes due to parastic capacitance between gate and drain during transitional response a spike noise which imparts change in a signal of corresponding photodetector Sn. This noise increases when the MOS transistor TGn is driven by higher power source voltage.
There is generally utilized a technology where the electric signal from the array 1 of photodetectors is voltage-converted so as to avoid reduction of output due to external load capacitance and external load resistance.
The voltage-convertion is generally carried out by a source follower circuit or operational amplifier.
However, the operational amplifier needs a wide dynamic range in order to transfer the signal output from the photodetector without its distortion. The dynamic range of the operational amplifier varies depending on magnitude of the power source voltage. The higher the power source voltage, the wider the dynamic range. On the other hand, when increasing the power source voltage of the read-out circuit, a large noise is superposed on a signal line through parastic capacitance and gate-drain capacitance as described above to thereby reduce S/N ratio. Namely, in the conventional semiconductor image sensing device, when the power source voltage is set higher, the noise increases. When the power source voltage is set lower, the dynamic range is reduced to hinder performance.